In the construction of buildings with so-called dry walls, it is conventional to apply a joint compound to the joints of wallboard panels to conceal the joints and present a monolithic surface. It has also been conventional to use either a setting type of compound or a drying type of compound, or both, depending upon construction conditions such as the rate at which the joint must be ready for further treatment or handling, atmospheric moisture conditions, and others. Setting types of compounds are characterized by a material, such as calcium sulfate hemihydrate, which combines with some of the water in the mix to convert the hemihydrate form into the dihydrate form. Drying is incidental and in some cases undesirable. An example of a setting type of compound is disclosed in U.S. Pat. No. 3,297,601. On the other hand, drying type compounds do not combine with the water, but rely on the evaporation of the water to give them strength. That is, instead of having a hemihydrate filler, calcium carbonate is often used. Such drying types usually incorporate a binder such as polyvinyl acetate, which, when dry, contributes the primary stress strength needed by the compound. An example of a drying type is disclosed in U.S. Pat. No. 3,342,761.
A recent development by the United States Gypsum Company featured the elimination of the expensive ingredient mica which heretofore was essential to prevent cracking due to shrinkage. The plate-like character of mica particles provides lateral reinforcement to the matrix so that it can withstand rather high dry-out shrinkage without cracking. The elimination of mica was accomplished by the use of as-mined calcium sulfate dihydrate known as landplaster, ground so as to have a particular particle size distribution, as described and claimed in U.S. application Ser. No. 277,446 filed on Aug. 2, 1972, commonly owned with the instant application. That application was based upon the discovery that the landplaster in question did not cause cracking such as was common with mica-free fillers heretofore used.
In addition to calcium sulfate dihydrate mined from the ground, considerable amounts of so-called by-product gypsum are produced synthetically by various conventional chemical reactions or processes. In each of these cases, the by-product gypsum is either undesirable, due for example to the presence of impurities, or is of little value because it lacks a significant market. An example of an impure form of by-product gypsum is that produced by the phosphoric acid process. A different process known as the citric acid process produces, on the other hand, a gypsum the purity of which cannot be equaled by any other source of gypsum. It is further distinguished by crystals having a peculiar twinned structure, and a very high brightness also not achievable by any other source. Landplaster or terra alba, on the other hand, comprises particles each of which are aggregated crystals and are less bright, due in part to impurities.